Steel cord users request longer and longer lengths on spools in order to reduce the downtime of the costly installations using such cords.
For example steel cord that is used to reinforce the belt or the carcass of a tyre is unwound from a creel containing sometimes hundreds of spools. These cords are calendered parallel to one another in rubber thus forming a steel cord reinforced ply for further processing into a tyre. Replacement of the empty spools with full ones is a laborious task one seeks to minimise. This is achieved by using larger spools containing longer lengths of cord. However, steel cord manufacturers cannot always deliver each spool at the full length requested without any interruption because the filament lengths are not always multiples of the final creel length. Additionally, in the manufacturing of steel cord random breaks can occasionally interrupt the process. Breaks are due to imperfections in the steel filaments attributable to e.g. non-deformable inclusions already present in the raw material. Therefore, incomplete lengths are interconnected and rewound at the required length. Although such an interconnection is extremely rare it must be able to withstand the calender process problem-free, because failure of such a single connection on one spool may lead to the halt of the complete creel resulting in lost production time and scrapped material.
Another example where steel cords must process without interruption is when these steel cords are used as strands in a steel cable. During the final closing step, such strands are unwound at high speed from spools in a cabling machine. The strands follow a—sometimes complicated—path through the machine while being tensioned, twisted and bent. Again failure of the connection will lead to the complete stop of the machine and an irreparable cable interruption.
There are different methods known in the art to connect steel cords together:                One way is to swage a ferrule over both ends held end-to-end. Such a ferrule can be made of an easily deformable metal like a copper or an aluminium alloy. The disadvantage of this connection is that it is substantially thicker than the cord itself. The steel cord is guided over many wheels, over wear parts and through holes after being unwound. The ferrule gets easily caught by these guiding parts and breaks. Also the connection is much stiffer.        An alternative to the swaging method is to use a polymer sleeve. This sleeve can be glued or heat shrunk over the cord ends. Although this connection is more flexible, the diameter problem remains. In addition, the connection is only borderline strong enough to hold the tensile forces occurring during the process.        By far the most preferred connection for a steel cord is a weld such as described in WO 03/100164. A good weld is made by locally shortening the lay length at each steel cord end prior to butt welding them together. During welding a blob of molten steel forms in which all filaments coalesce. By preference the welding process is followed by a thermal annealing of the welding area. Although the strength of the cord containing a weld is significantly lower than the strength of the weld-free cord (usually one loses 50 to 60% of the cord strength at the weld) this is not an immediate problem to process the cord further. The diameter of the weld can be controlled by hammering. The norm is that the diameter at the weld must not be larger than 1.10 times the diameter of the cord.However one major drawback to the welding method remains. Steel cords are made of steel filaments that are twisted together. The steel filaments are cold drawn and due to this strain hardening process their tensile strength (breaking load per unit area) is greatly increased. This increase finds its origin in the changed metallurgical structure of elongated perlitic grains wherein dislocations are rearranged so as to prevent crystallographic planes from gliding over one another. By making a weld this structure is locally disturbed and an annealed martensitic structure is formed in the weld. Although such a structure is strong it is more brittle. In addition there is a transition region between annealed martensitic and cold-drawn perlitic where the filaments tend to break off easily upon bending. So during handling of the cord, it is not the weld that gives in, but it are filaments that crack very close to the weld. While such a filament break may not lead to a cord breakage, the loose filament end will disentangle from the cord and can be stripped off, leading to a complete process stop.        
This ‘filament breaking problem’ occurs with all kinds of steel cords but is particularly severe when so called ‘open cords’ are welded. Such open cords comprise filaments that are preformed in one or another way (e.g. helically preformed as described in U.S. Pat. No. 4,258,543, polygonally preformed as per WO 95/16816 or double crimped according EP 1036235 B1). Due to the preforming the filaments can move relative to one another as they are not always in contact with one another. When now such a cord is led through a narrow-fitting hole or is squeezed while being encapsulated in the rubber, some filament may accumulate an overlength with respect to the other filaments. Such a filament visibly separates from the other filaments and shows as an eyelet rotating around the cord as the cord evolves. After a while the overlength on one filament may disappear followed by the formation of an eyelet on another filament. This phenomenon is known in the art as ‘sleeving’. Such a sleeving on itself is relatively harmless and is intrinsic to the open cord structure. However, when sleeving occurs at a weld, it becomes catastrophic as the overlength is pulled to the weld where all filaments are molten together. The filament cannot longer move and cracks between the restraining hole and the weld. The filament is stripped off and forms a wire nest. If the process is stopped soon enough the damage can be contained. If not, the cord will break and entangle cords leading to a complete creel mess.
Prior to the proposed invention, it was not possible to supply open cords that contained welds. Although most welds went through without giving ‘filament breaking’ problems, the ‘survival rate’ was never high enough to enable a stable and economic process. With the inventive connection, the ‘filament breaking problem’ is a problem of the past.